Referring to FIG. 1, optical devices disposed in a carriage 1 of an image scanning apparatus is schematically shown. The light reflected or transmitted from a scanned object (not shown) is incident into the carriage 1 via an optical grid plate 10. By means of a lens set 11, the light is focused on a charge-coupled device (CCD) 12 for exposure, thereby implementing photoelectric conversion. In the figure, arrows are used to schematically indicate the light path.
A front view of a typical optical grid plate 10 is schematically shown in FIG. 2. The optical grid plate 10 has a light-transmission aperture 101 arranged in the optical path. In this figure, the symbol “x” indicates the light enters the carriage behind the plate 10 from the light-transmission aperture 101. The aperture 101 is shaped as a dumbbell as shown. That is, both side portions are larger than the middle portion in area. The aperture 101 with such shape facilitates the compensation of the relatively weak light intensity emitted by the end portions of a linear light source so as to unify the light amount reaching the lens set 11 at all positions.
Generally, the resolution of an image scanning apparatus is adjustable, and the conditions of the image scanning apparatus are tuned according to a preset resolution. For example, an image scanning apparatus is adjustable between a high resolution of 600 dpi and a low resolution of 300 dpi. As is understood by those skilled in the art, the movement of the carriage under the selected resolution 600 dpi is slower than that under the selected resolution 300 dpi. Accordingly, for each signal scan line, it is exposed less under the high resolution than that under the low resolution. If the conditions of the image scanning apparatus are preset to comply with the requirement of that high resolution, i.e. if the exposure is perfect for the high resolution scanning state, the exposure will be insufficient for the low resolution scanning state. As such, the signal-to-noise ratio of the photoelectric converted output signal will be undesirably low under the low resolution scanning state, and thus the scanning quality is adversely affected. On the other hand, if the conditions of the image scanning apparatus are preset to comply with the requirement of that low resolution, i.e. if the exposure is perfect for the low resolution scanning state, overexposure situation will be rendered for the high resolution scanning state, which might result in distortion of the scanned image.